It is well known that X-rays can be diffracted from crystalline solids to produce diffraction patterns whose interpretation gives information on the atomic structure of such crystalline materials. It is also well known that X-rays can be used to excite fluorescent emissions from materials, and that the fluorescent emissions can be used to provide information on the composition of the materials exposed to the incident X-rays. Additionally, it is well known that X-rays can be reflected from smooth surfaces if the reflection angle between the incident X-rays and the surface is less than some critical value. Indeed, the use of the reflection of X-rays from smooth surfaces is a basic feature of the design of X-ray telescopes. Because the image forming quality of such telescopes depends in part on the smoothness of the reflecting surface, studies have been made of the influence of surface topography on the reflected X-ray beam.
The apparatus and method of the present invention relate to the use of X-ray reflection to determine the chemical composition of the surface from which the X-rays are reflected. Because the X-ray beam is being reflected and not diffracted, it is not required that the reflecting surface be crystalline in nature and the chemical composition of either crystalline or amorphous surfaces can be equally well determined. By the choice of particular incident X-ray wavelengths it is possible to analyze the composition of surfaces composed of any of the known elements, including elements of low atomic number. When X-ray fluorescent methods, such as that taught by Dorenbosch and Zingaro in U.S. Pat. No. 3,011,060 are used for such analyses, there will be elements of low atomic number, such as carbon (atomic number 6) and nitrogen (atomic number 7) which cannot be easily detected because the fluorescent X-rays from such atoms are very weak and are of very long wavelength. The X-ray wavelength characteristic of fluorescent sulfur, atomic number 16, is already approximately 5.4 angstroms while the X-ray wavelength characteristic of carbon, atomic number 6, is more than 44 angstroms. The fluorescent wavelength characteristic of lithium, atomic number 3, is 228 angstroms. The present invention is not so limited because it does not depend upon the excitation of the fluorescent radiation. Additionally, the present method is capable of analyzing much thinner layers than are other X-ray methods and layers of only 100 angstroms can be analyzed due to the fact that the X-rays are being reflected and so do not penetrate as significantly into the surface being analyzed as do normal X-ray analytical procedures.